Ok this is a weird one.. it's a 94 LS with no trac, ecu and gauge cluster have been repaired by prev owner(professional repair, damaged traces are repaired and look perfect, thin shiny straight traces and new caps), plugs, wires, coils, oxygen sensors, most vac lines replaced.

Sometimes when I try to accelerate it will feel like I have only 100 hp, and others it will feel closer to the 250 or so it's supposed to have. Now it wont decrease when I accel, but sometimes it will "click" into full power around 2500rpm. Now I put an AEM wideband on it and the air to fuel ratio seems just fine. Not too lean and nit too rich. Closed loop is about 15.5 both with I have lots of power and when I have a loss.. same thing with open loop, it's between 10-12:1.

I'm at a loss here.. it's confusing me, I feel like I'm chasing my tail. Anyone have some ideas??

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I know it has been done, but sure sounds like ECU capacitors - many times that repair is done using the incorrect type of capacitors (not low ESR type from a good company, because most repair techs don't know any better and think any new caps will do)

please do us a favor and pull your ECU and photograph the caps used and try to identify them or else yamae and I can identify them for you if the pics are good enough - you want to note all of the numbers/letters on the caps, to make sure they are the correct brand, type, polarity, value, etc. and report those numbers here (example: Rubycon 10uF - 50V - series ZLH)

yes, it has been documented on all models through year 2000, and I will not be surprised if the next generation is not affected (2001-2006), perhaps still a bit early for 2001 and up, to exhibit the problem in large frequency yet.

98-00 are starting to sharply increase in reported occurrence, 90-97 are close to epidemic! (it's like the flu - lol)

yes, it has been documented on all models through year 2000, and I will not be surprised if the next generation is not affected (2001-2006), perhaps still a bit early for 2001 and up, to exhibit the problem in large frequency yet.

98-00 are starting to sharply increase in reported occurrence, 90-97 are close to epidemic! (it's like the flu - lol)

it's been seen on '98-00 but is not nearly as widespread as LSCowboyLS has repeatedly made it seen. there have been a mere handful of '98+ cars with confirmed ECU issues reported. it has been Predicted that 98+ cars will be having problems in the near future.

OP: before deciding that the ECU is a factor, what is the state of your vehicle's ignition and fuel system? When was the car last tuned up?

It would be interesting to hear of the JDM market issues from yamae. The Japan seasons are pretty brutal just as some regions in the states. The mild weather cars might do a bit better without the extremes to deal with.

also, the car seems to shift entirely too early sometimes (2000 rpm at half throttle) for only one shift but then holds the next gear out. and occasionally i feel as if there is a flat spot in the throttle.

sometimes the caps will begin to fail prior to leaving any visual evidence

this gave me fits on my car, because I had looked at the ECU mainboards and caps with a magnifying glass even just a month before, and everything looked perfect, and the caps were already malfunctioning - luckily I had not reinstalled the glove compartment yet, and something made me go back and replace all of them any way.

It would be interesting to hear of the JDM market issues from yamae. The Japan seasons are pretty brutal just as some regions in the states. The mild weather cars might do a bit better without the extremes to deal with.

Yes, our seasons are really brutal. I was pretty much amazed at the mild weather there in Southern California when I had started to live and work there in late 90s. But I know the summer in Arizona and Texas. I also know the 0 degrees F coldness in Detroit. We almost have them in a small island.

Most of UCF10s and 11s have gone now and I don't see them at all these days. According to repair shop owners I know, there used to be many ECU failed UCF10s and 11s. Regarding UCF20s and 21s manufactured before June 97, ECUs are now failing next to next. Regarding models after July 97, there are some but not too many. As far as I know, there are only 2 failed ECUs. One can be seen here. Sorry it is all written in Japanese but I am sure you can assume the situations by the photos, alphabets and figures in the page.

My 98 Celsior had a minor ECU problem caused by a metal film resistor increasing the resistance. Those capacitors were still OK but I dare decided to change them as a preventive maintenance.

ECUs for after July 97 have totally different new circuits. New processors consume less energy and all the new DC regulators have bigger margin and ceramic capacitors of a bigger capacitance are widely used in parallel with electrolytic capacitors. These new circuits reduce ripple currents to those Nichicon PF(M) capacitors and they are less stressed. In addition to new circuits and younger age of Nichicon PF(M) capacitors, the failure rate of ECUs after July 97 are less, I think. But those contain infamous liquid "quaternaty ammonium salt" in it and they are prone to fail after many years of use.

when I was an auto electrical problem technician in Phoenix, Arizona; we saw a really bad and very common LS400 problem related to the high heat there that can climb to over 122F (50C)

the power door lock assemblies would fail, and the new modules were $400 per door and all doors were affected and the final bill to the customer was $1600 just for the parts, and then labor on top of this - it is interesting because this is a very uncommon LS400 problem in places other than southern/western Arizona, Death Valley, California, and Las Vegas

I worked one day in Phoenix in 1995 where the temperature at our shop climbed to 133F (56C) in the shade!!

many auto manufacturers of USA, Japan, Korea, and Germany have test tracks in southern Arizona in towns such as Wickenburg and Wittmann to use as heat torture test tracks for there new car designs!

sometimes the caps will begin to fail prior to leaving any visual evidence

this gave me fits on my car, because I had looked at the ECU mainboards and caps with a magnifying glass even just a month before, and everything looked perfect, and the caps were already malfunctioning - luckily I had not reinstalled the glove compartment yet, and something made me go back and replace all of them any way.

Oh i meant i will check which caps are installed. it looks fairly recent that they have been replaced, but they could be the wrong type.